1. Field of the Invention
The present invention relates to the improvement of a master cylinder used for various braking devices, and so on, of an automobile.
2. Description of the Related Art
As a master cylinder of the kind relating to the present invention, there is available a tandem brake master cylinder MC like that shown in FIG. 5. This tandem brake master cylinder MC is loaded in front of a not-shown brake power assist (brake booster) MV. The cylinder housing 1 of the master cylinder MC is composed of a body section 2 having an opening part in one end, and a cap 3 connected (prevented from being pulled out by a retainer) to the opening part of the body section 2.
A sleeve 8 is fitted in the cylinder housing 1. A primary piston 4 and a secondary piston 5 are inserted through the cap 3 and freely slidably disposed to be guided and supported by a piston guide section 3a formed in the cap 3 and the sleeve 8.
Inside the cylinder housing 1, a first pressure chamber 9 is formed by the primary piston 4, the secondary piston 5 and the cap 3. A second pressure chamber 10 is formed by the secondary piston 5 and the inner wall surface of the body section 2.
Recessed sections 4a and 5a opened in tip sides are respectively formed in the primary piston 4 and the secondary piston 5. Small holes 4b and 5b are respectively provided in peripheral walls forming the recessed sections 4a and 5a. In a brake releasing position, a replenish liquid passage is formed to be leading from an oil reservoir 23 to the first and second pressure chambers 9 and 10.
Spring mechanisms 11 and 12 are respectively provided in the first and second pressure chambers 9 and 10. The spring mechanisms 11 and 12 are disposed respectively between the recessed section 4a of the primary piston 4 and the secondary piston 5 and between the recessed section 5a of the secondary piston 5 and the inner wall surface of the body section 2. These spring mechanisms 11 and 12 press the primary piston 4 and the secondary piston 5 respectively in returning directions.
The spring mechanism 11 is composed of a return spring 15, a guide pin 16 having a large head portion 16a, a spring retainer 17 shaped like a derby hat (alternatively, shaped like a flange cylinder), and a guide pin fixing member 18. The return spring 15 is provided to be contracted between the peripheral portion of the guide pin fixing member 18 and the peripheral flange portion 17a of the spring retainer 17. One end of the return spring 15 is fitted in an expanded diameter portion formed in the bottom surface of the recessed section 4a of the primary piston 4. The guide pin 16 is disposed so as to be away from the bottom surface of the recessed section 4a by a predetermined length and protruded in a piston axial direction. The guide pin fixing member 18 is fixed in a fixing groove formed in the end part of the guide pin 16. The spring retainer 17 includes a hole 17b provided in the a top part of its derby hat shape. This hole 17b is freely slid to the guide pin 16 inserted into the hole 17b within the range of a predetermined stroke (range slightly exceeding the returning limit of the primary piston 4) and engaged with its large head portion 16a. The spring mechanism 12 includes only a return spring.
With the master cylinder MC thus constructed, when the primary piston 4 and the secondary piston 5 are respectively pushed into the first and second pressure chambers 9 and 10 against the pressing forces of the spring mechanisms 11 and 12 in a left direction in the drawing, liquid pressure in each of the pressure chambers 9 and 10 is increased and hydraulic liquid is forcibly fed from feed ports 13 and 14 to a not-shown braking system.
As shown in FIGS. 6(a) and 6(b), in the peripheral portion 17a of the spring retainer 17 of the spring mechanism 11 included in the master cylinder MC, there are integrally formed a plurality (e.g., four) of tip portions 17c which are protruded outward in a radial direction. A wide guide groove 20 having a constant width is provided in the sleeve 8. This guide groove 20 is formed in an axial direction for inserting and engaging the tip portion 17c so as to penetrate the inner and outer peripheral surfaces of the sleeve 8. The guide groove 20 includes a stopping section 20a formed in a position slightly exceeding the returning limit of the primary piston 4 from the end of the sleeve 8. The tip portion 17c is brought into contact with the stopping section 20a and stopped therein.
According to the above-described master cylinder MC relating to the present invention, during brake operation, the deformation of the sleeve 8 can be prevented, machining of the guide groove 20 can be facilitated, and an ineffective stroke can be freely set in combination with the brake power assist (brake booster) MV without regulating the returning limits of the primary and secondary pistons 4 and 5. See Japanese Patent Application No. 8-187081 (No. 187081/1996)!.
However, there are problems inherent in the master cylinder MC. One problem is concerned with the spring retainer 17 for preventing the pulling-out of the primary piston 4. Specifically, when the primary piston 4 is strongly pulled, the tip portion 17c formed in the radial direction of the peripheral portion 17a of the spring retainer 17 is deformed inward and bent. Consequently, the primary piston 4 may be pulled out.
Another problem is concerned with the formation of a low projecting section in the rear end surface of the secondary piston 5, which is used for positioning the spring retainer 17. Since a portion brought into contact with the spring retainer 17 is only formed to be tapered, the deformation of the tip portion 17c cannot be prevented.